                a) Field of the Invention        
The present invention relates to a lens system for microscope, and more specifically an apochromat objective lens system for microscope which has a magnification on the order of 4× to 40× and aberrations favorably corrected within a range from a visible region to a near infrared region.                b) Description of the Prior Art        
In markets of research fields associated with biology, there have recently been posed requirements such as those which are described below.
First, in a market of a fluorescence observation method by which fluorescence has been observed in a near infrared region utilizing pigments which emit fluorescence having wavelengths longer than those conventionally used, there are requirements for optical systems which exhibit favorably imaging performance in the near infrared region.
Furthermore, in a field of a near infrared DIC observation method which has recently been utilized, there are requirements for favorable imaging performance in the near infrared region.
Furthermore, in a field of an observation of multiple photons typically represented by two photons in which a specimen is excited by rays having long wavelengths and the specimen is observed with visible rays, it is desired to obtain a small departure between locations of focal points on the specimen at wavelengths in the visible region and a wavelength in the near infrared region.
Moreover, in a field of a simultaneous observation by the fluorescence observation method and the near infrared DIC observation method, it is desired to obtain a small departure between locations of focal points on a specimen at the wavelengths in the visible region and the near infrared region.
For reasons described above, it is desired to obtain an apochromat objective lens system for microscope in which aberrations are favorably corrected within the range from the visible region to the near infrared region.
As objective lens systems for microscopes in which aberrations are favorably corrected within the range from the visible region to the near infrared region, there are conventionally known those which are disclosed by the following literatures:
Japanese Patent Kokai Application No. Sho 62-49313
Japanese Patent Publication No. Hei 7-104488 (Japanese Patent Kokai Application No. Hei 4-26813)
Japanese Patent Kokai Application No. Hei 6-175034
Japanese Patent Kokai Application No. Hei 11-174338
Japanese Patent Kokai Application No. 2003-167199
Out of conventional examples disclosed by the above-mentioned literatures, objective lens systems disclosed by Japanese Patent Kokai Application No. Sho 62-49313 are an objective lens system which has a magnification of 50× and an NA of 0.5, and an objective lens system which has a magnification of 60× and an NA of 0.6.
Furthermore, objective lens systems disclosed by Japanese Patent Publication No. Hei 7-104488 are an objective lens system which has a magnification of 5× and an NA of 0.14, an objective lens system which has a magnification of 10× and an NA of 0.26, an objective lens system which has a magnification of 20× and an NA of 0.4, an objective lens system which has a magnification of 50× and an NA of 0.42, and an objective lens system which has a magnification of 100× and an NA of 0.5.
Furthermore, disclosed by Japanese Patent Kokai Application No. Hei 6-175034 is an objective lens system which has a magnification of 50× and an NA of 0.45.
Furthermore, disclosed by Japanese Patent Kokai Application No. Hei 11-174338 are an objective lens system which has a magnification of 5× and an NA of 0.13, an objective lens system which has a magnification of 10× and an NA of 0.21, an objective lens system which has a magnification of 20× and an NA of 0.35, an objective lens system which has a magnification of 50× and an NA of 0.4, and an objective lens system which has a magnification of 100× and an NA of 0.5.
Moreover, disclosed by Japanese Patent Kokai Application No. 2003-167199 is an objective lens system which has a magnification of 100× and an NA of 0.7.
The objective lens systems mentioned as these conventional examples are used for observing and inspecting semiconductor integrated circuits and liquid crystal panels. These objective lens systems correct aberrations within ranges to near infrared rays for repairing wiring defects using an YAG laser (1064 nm). However, these objective lens systems which are configured to observe the semiconductor integrated circuits and the liquid crystal panels have long working distances and therefore small numerical apertures. Accordingly, these objective lens systems are unsuitable for use as objective lens systems for observation of cells, proteins, DNAs and the like which must be observed as highly resolved images and bright images.